Updates the actions/checkout action used in the GitHub Actions workflow to its newest major version.

Changes in actions/checkout:

v3.1.0

• Use @actions/core saveState and getState
• Add github-server-url input

v3.0.2

• Add input set-safe-directory

v3.0.1

• Fixed an issue where checkout failed to run in container jobs due to the new git setting safe.directory
• Bumped various npm package versions

v3.0.0

• Update to node 16

Still using v2 of actions/checkout will generate some warning like in this run: https://github.com/servo/rust-smallvec/actions/runs/3783768281

Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/checkout@v2, actions-rs/toolchain@v1

The PR will get rid of those warnings for actions/checkout, because v3 uses Node.js 16.